1. Field of the Invention
This invention relates to aqueous polyesters for high-solids stoving lacquers which contain urethane, carboxylate and hydroxyl groups and only small quantities of organic auxiliary solvents and volatile neutralizing agents, to a process for the production of these polyesters and to their use.
2. Description of the Prior Art
It is known that water-based binders which have a relatively high solids content and contain little or no organic auxiliary solvent can be produced by using internal or external nonionic emulsifiers (cf. for example Die Angewandte Makromolekulare Chemie 123/124 (1984), pages 285 to 306 and pages 437 et seq) and D. H. Solomon, "The Chemistry of Organic Film Formers", John Wiley & Sons, Inc. (1967) pages 280 et seq. Since these emulsifiers remain in the film even after curing, they can adversely affect the properties of the coatings, including their resistance to water, detergents and weathering. Another problem of nonionic emulsions of the type in question is their generally poor stability, for example, to shear stressing during the production of lacquers, such that they tend to coagulate. In addition, it is not possible to obtain high-gloss coatings from these products.
Water dilutability of polymers can also be obtained by chemically incorporated, base-neutralized acid groups, more particularly carboxyl groups, or by chemically incorporated acid-neutralized tertiary amino groups (cf. for example DE-OS No. 2,749,692, EP-A No. 218,906 or L. Dulog, Die Angew. Makro. Chemie 123/124 (1984), pages 437 et seq). During the curing of coatings containing such binders, the films become insoluble in water through dissociation of the hydrophilic salt groups and evaporation of the volatile neutralizing agents. However, depending on the type of neutralizing agent used and the hardening conditions, large quantities of neutralizing agent may remain in the film.
If the predominant quantity of neutralizing agent remains in the film, the resulting hydrophilic salt groups represent potential points of attack, for example for moisture, such that the quality of the coatings can be adversely affected. By contrast, evaporation of the amine in relatively large quantities results in environmental concerns, especially when relatively large quantities of volatile, organic auxiliary solvents are also used.
To reduce environmental pollution, expensive absorption or incineration plants are necessary.
For these reasons, there is a need for water-based coating compositions which contain very little organic solvent and, in addition, only require a small quantity of volatile neutralizing agents.
In addition, a water-based coating composition is expected to have a high solids content in water. This is desirable for energy-saving reasons, for example, reduced transport costs and a relatively low heat demand for evaporating the water during the curing of the binders. In addition, more favorable application or film properties, including greater layer thicknesses, can be obtained. Dry film thicknesses of 50 to 70 .mu.m are generally difficult to achieve with water-based binders because there is a pronounced tendency towards the formation of boiling-induced bubbles, craters and other film defects. These defects are normally eliminated or reduced by the addition of volatile high-boiling solvents, organic auxiliary solvents or similar additives, although their addition makes the binders less environmentally acceptable.
Water-based binders are also expected to provide high resistance to hydrolysis and storage although this is not always the case, particularly with low-solvent binders.
Highly flexible coatings are being increasingly used in many fields of application, for example, in industrial coatings, in automotive non-chip coatings and fillers, for crack-covering coatings, etc. Newly developed binders should enable highly flexible coatings to be produced, whether in combination with crosslinking resins or as a flexibilizing additive to existing binder compositions having insufficient flexibility.
Accordingly, an object of the present invention is to provide a water-based binder for high-solids stoving lacquers which satisfy the previously stated requirements. The coatings produced with this binder should contain less than 6.0% by weight, preferably less than 4.5% by weight and more preferably no organic solvent; contain less than 1.1% by weight, preferably less than 0.9% by weight, of neutralizing agent; and have a solid contents of .gtoreq.50% by weight, preferably .gtoreq.54% by weight. In addition, the binder should contain free hydroxyl groups to allow crosslinking reactions at temperatures &gt;110.degree. C. The binders according to the invention should be suitable for the production of flexible coatings and, in addition, should be capable of being applied without difficulty in considerable layer thicknesses. In combination with crosslinking resins to form stoving lacquers, the binders according to the invention should be stable in storage.
It has now surprisingly been found that these objectives may be achieved with the binders of the present invention described in more detail hereinafter.